Search for exclusive photoproduction of $Z_c(3900)$ at COMPASS

TL;DR

This study searches for the exclusive production of the $Z_c(3900)$ hadron state via virtual photons at COMPASS, setting upper limits on its production cross-section and decay branching ratio, contributing to understanding its nature.

Contribution

First experimental search for direct exclusive production of $Z_c(3900)$ via virtual photons, providing upper limits on production and decay ratios at COMPASS.

Findings

No significant $Z_c(3900)$ signal observed.

An upper limit of $3.7 imes 10^{-3}$ established for the product of branching ratio and cross-section ratio.

Data from 2002-2011 used to set constraints on $Z_c(3900)$ production.

Abstract

The $Z_{c}(3900)$ hadron state has been found by the BES-III and Belle experiments in the decay of the hadron state with higher mass. The first attempt to search for the direct exclusive production of the $Z_c^{\pm}(3900)$ hadron by virtual photons has been performed in the reaction $\mu^+ N \rightarrow \mu^+ N' Z_c(3900)^{\pm} \rightarrow \mu^+ N' J/\psi \pi^{\pm}$ at COMPASS \cite{Adolph:2014hba}. The data cover the range from 7~GeV to 19~GeV in the centre-of-mass energy of the photon-nucleon system. The full COMPASS data set collected with a muon beam between 2002 and 2011 has been used. An upper limit for the ratio $BR(Z_c^{\pm}(3900)\rightarrow J/\psi \pi^{\pm} )\times (\sigma_{ \gamma~N \rightarrow Z_c^{\pm}(3900)~ N} /\sigma_{ \gamma~N \rightarrow J/\psi~ N})$ of $3.7\times10^{-3}$ has been established at the confidence level of 90\%.